The Home Phoneline Network Alliance (HPNA) has defined a set of standards of how to manage a data network transmitted over the phone lines in a home (rather than along separate data lines).
In HPNA v.2, the network devices are asynchronous network devices, sending messages when they sense that the network medium was available. The asynchronous network devices have collision detection (CD) abilities and can detect if another network device is transmitting at the same time that they do. If so, both network devices back off for a random amount of time after which, they retransmit the packet. Such a back off system works well for a small number of network devices, but, since the back offs are for a random amount of time, the HPNA v.2 network devices cannot guarantee quality of service for services with specific timing requirements, such as voice over IP (VOIP) or video downloads.
The most recent standard, HPNA v.3, defines a network that attempts to guarantee quality of service (QoS) requirements while being retro-compatible with the previous network devices. HPNA v.3 requires that its network devices operate synchronously, each network device being allowed to transmit only during the timeslots assigned to it. For this, at least one of the network devices has a scheduler to assign the timeslots so as to guarantee media resources to network devices, to prevent collision between multiple network devices using the same line and to ensure quality of service. The asynchronous network devices, either an HPNA v.2 network device or a non-HPNA network device, do not know the boundaries of the timeslots and thus, transmit whenever they sense that the medium is available. HPNA v.3 includes rules for handling such interruptions in the transmissions so as to minimize the effect such interruptions have on the quality of service.